This is a simple Potion Creation Success Rate calculator based on this formula.

Success Rate = PR x 1 + PP x 3 + Job Level x 0.2 + (DEX + LUK+ (INT/2)) x 0.1 + a + 5(%) (Minimum Success Rate)

Leaving your homunculus out will increase rates by approximately 5%.
Apparently this has to do with the Homunculus skill Change Instruction's level.
a figures:

• Red Yellow White pots :15
• Alcohol: 5
• Acid Bottle, Plant Bottle, Marine Sphere Bottle, Bottle Grenade: -5
• Blue Potions Embryo, Various Resist Potions, Aloevera, Anodyne, Red Slim Bottle: -10
• Yellow Slim Potions: -12.5
• White Slim Potion, Glistening Coat: -15